Direct atomic-scale confirmation of three-phase storage mechanism in Li₄Ti₅ O₁₂ anodes for room-temperature sodium-ion batteries

Room-temperature sodium-ion batteries attract increasing attention for large-scale energy storage applications in renewable energy and smart grid. However, the development of suitable anode materials remains a challenging issue. Here we demonstrate that the spinel Li₄ Ti₅ O ₁₂, well-known as a 'zero-strain' anode for lithium-ion batteries, can also store sodium, displaying an average storage voltage of 0.91 V. With an appropriate binder, the Li₄Ti₅ O₁₂ electrode delivers a reversible capacity of 155 mAh g -1 and presents the best cyclability among all reported oxide-based anode materials. Density functional theory calculations predict a three-phase separation mechanism, 2Li₄ Ti ₅ O ₁₂+₆Na + +₆e - â†"Li₇Ti₅ O₁₂ +Na 6 LiTi 5 O 12, which has been confirmed through in situ synchrotron X-ray diffraction and advanced scanning transmission electron microscope imaging techniques. The three-phase separation reaction has never been seen in any insertion electrode materials for lithium- or sodium-ion batteries. Furthermore, interfacial structure is clearly resolved at an atomic scale in electrochemically sodiated Li₄Ti₅ O₁₂ for the first time via the advanced electron microscopy.